As systems, such as the multimedia entertainment, communications, process control, and diagnostic systems utilized by the automotive and aerospace industries, become more complex, a need arises for additional devices to communicate, either with each other or with a central controller or the like. Historically, these systems included dedicated wiring extending between the various devices in order to support communications therebetween. As systems have become more integrated and the communications requirements have been increased, the amount of dedicated wiring that would be required has become excessively large, both in terms of the space required for the wiring and the cost of the wiring and the attendant installation. Moreover, as the amount of dedicated wiring increases, the overall complexity of the system also generally increases, as well as the likelihood that some portion of the wiring might be damaged or broken during or following installation.
As such, network systems have been developed to provide a common communications path between a plurality of network devices. In automotive and aerospace applications, for example, a network system can be utilized to monitor various components and to collect diagnostic and status information. In this regard, diagnostic and status information relating to the strain, acceleration, pressure and/or temperature to which the various components are subjected may be collected and analyzed.
By way of further example, a network bus architecture is currently being developed to support communications and the delivery of multimedia information to the occupants of a vehicle, such as an automobile, minivan, sports utility vehicle, aircraft, boat or the like. Advantageously, this network bus would transport the audio signals, including streaming audio signals, produced by one or more of a radio, a cassette tape player, a compact disc player or the like to selected speakers or headphone jacks throughout the vehicle. Similarly, the network bus may support voice and data communications with a cellular telephone carried by an occupant of the vehicle, as well as communications with a laptop computer, a handheld computing device or the like. In addition, the network bus may transmit video signals, including streaming video signals, from a television receiver, a videocassette recorder or other video source to one or more video monitors. In addition, the network bus may transmit sensor and actuator signals to and from devices such as drivetrain devices, passive restraint devices, crash avoidance devices, drive-by-wire devices, or the like. Further, the network bus system may transport information related to diagnostic performance of the vehicle.
In addition to the variety of devices that are connected to a network bus, one or more controllers are also generally connected to the network bus for sending commands to the various remote devices and receiving data from the remote devices. Among other things, these commands specify the manner in which the various devices are to function including the manner in which the various devices are to transmit information over the network bus.
Traditionally, networks of the type described above have transmitted data in analog format. Unfortunately, analog signals are susceptible to noise introduced into the signals during data transmission. Given that many of the transmitted signals have a low amplitude to start with, this noise can corrupt the signal and decrease the signal to noise ratio to levels that cause loss of resolution in the signal. Further, as many of these network devices are scattered some distance from the controller, the electrical lines connecting the network devices to the controller may be sufficiently long to cause signal degradation due to DC resistance in the wiring.
In light of these shortcomings, it would be advantageous to utilize digital networks. But, many conventional digital networks suffer from a variety of problems themselves. For example, many existing digital networks operate according to complicated protocols which require each network device to have a relatively high level processor, thereby increasing the cost of the network devices. Complicated protocols also introduce overhead into the messages on the bus that are not necessary for data acquisition and control. This overhead can severely limit the number of data samples that can be transmitted on the bus. These networks also have other problems. For example, they generally do not support both acquisition and control, and they typically only support networks that extend over relatively short lengths. Further, these networks typically have bulky network device interfaces, slow network data rates and/or a low network device count. Additionally, many computer systems that include digital networks do not operate in a time-deterministic manner. As such, these computer systems generally lack the capability to schedule a trigger on event command to the network components that repeats or is interpreted and executed with any precision timing.
Further, some conventional network systems operate in a master/slave configuration, where the bus controller controls all communication on the network bus. In these network systems, the bus controller uses an operation schedule, (sometimes referred to as a command schedule), that includes commands related to either one or a group of network devices and the timing for communicating with the network devices. As such, the bus controller mandates all or substantially all timing for communications.
Although placing the bus controller in control of all communications on the network bus is advantageous as it eliminates communication conflicts on the network bus, there may be some drawbacks to this configuration. Specifically, in some instances, timing for sending commands to various network devices on the network is dictated by an event of interest in a process with which the network system is associated. For example, in many data acquisition and control systems, it is typically desirable to acquire data or perform actions with accurate timing and control. In these systems, it is crucial or at least beneficial to synchronize commands sent by the controller to the network devices with the occurrence of an event of interest of a process to which the network system is associated.
As an example, in regard to automobiles, it may be advantageous to provide a network system that, among other things, maintains precise engine control. In this example system, knowledge of the rotation of the crankshaft of the engine may be a critical factor for data acquisition and control. The rotation of the crankshaft may be referenced according to a once per revolution mark referred to as a top dead center (TDC) position In this system, to accurately acquire data and control the performance of the engine, the TDC position should be used to synchronize commands from the bus controller to various network devices associated with controlling the engine.
To synchronize the bus controller with an event of interest, some conventional network systems include a sensor or similar device connected to the process for providing an indication to the bus controller when the event has occurred. (In the above-mentioned example, the network may include a sensor that detects each time a reference point on the crankshaft is at the TDC position.). However, because communications on the network bus are controlled by the bus controller and because it is not predeterminable before hand when the event will occur, the sensor is not part of the network system. Instead, it is connected to the bus controller by a separate, dedicated line, so that when it transmits an indication that the event of interest has occurred, it will not disrupt communication on the network bus.
As stated above, however, an important advantage of network systems is the replacement of dedicated wiring with a common network bus. The requirement of the additional dedicated wiring by the above-described conventional network system is not consistent with this goal and suffers from problems, such as added cost of the wiring and the attendant installation, as well as the likelihood that some portion of the wiring might be damaged or broken during or following installation.